Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large scale manufacture of integrated circuits. A typical ion implanter comprises three sections or subsystems: (i) a terminal for outputting an ion beam, (ii) a beamline for directing and conditioning the beam output by the terminal, and (iii) a target chamber which contains a semiconductor wafer to be implanted by the conditioned ion beam. The terminal includes a source from which a beam of positively charged ions is extracted. The beamline components adjust the energy level and focus of the extracted positively charged ion beam on its way toward the target wafer.
A problem encountered in the use of such an ion implanter is that of wafer charging. As the positively charged ion beam continues to impact the target wafer, the surface of the wafer may accumulate an undesirable excessive positive charge. This accumulated positive charge is often non-uniform in its distribution across the surface of the wafer. Resulting electric fields at the wafer surface can damage microcircuitry on the wafer. The problem of accumulated surface charge becomes more pronounced as implanted circuit elements become smaller, because smaller circuit elements are more susceptible to damage caused by the resultant electric fields.
A known solution to the wafer charging phenomenon is the use of a plasma shower. A typical plasma shower includes an arc chamber in which an inert gas is ionized to produce a plasma comprised at least partially of low energy electrons, and a plasma chamber into which the plasma is extracted from the arc chamber and through which the ion beam passes. The plasma chamber contains a filament which is electrically heated so that it thermionically emits high energy electrons into the plasma chamber. The high energy electrons collide with inert gas molecules to create the plasma which includes low energy electrons capable of being trapped within the ion beam. The trapped low energy electrons thereby neutralize the net charge of the beam which in turn reduces the positive charge accumulation on wafer as the ion beam strikes the wafer surface. Such a system is shown in U.S. Pat. No. 4,804,837 to Farley, assigned to the assignee of the present invention and incorporated by reference as if fully set forth herein.
Plasma showers for neutralizing positively charge ion beams typically utilize helical coil or "pigtail" type filaments, having a uniform cross section along the entire length thereof, to produce thermionic emission of electrons (see, e.g., U.S. Pat. No. 4,463,255 to Robertson et al. and U.S. Pat. No. 5,399,871 to Ito et al.). Such helical filaments, however, present several operational impediments. For example, the uniform cross section of the coil filament provides for a uniform resistivity along the length thereof, which provides a corresponding uniform heat generation along the length thereof (i.e., as much heat is conducted by the legs or ends of the filament as is conducted by the center of the filament). As such, the ends (legs) of the filament provide a significant portion of the total heat conductivity/dissipation of the filament. In addition, the uniform cross section of the pigtail filament results in the establishment of a temperature gradient along the length of the filament, from the midpoint of the coil (hottest) to either leg of the coil (coolest).
Because electron emission is space charge limited, a large emission area of the filament is required to produce suitable electron emission. In order to raise a sufficient area of the filament to the temperature required to achieve thermionic electron emission, a "hot spot" is necessarily created within the temperature gradient of the filament, typically near the midpoint of the length of the coil. The electron emission rate near this midpoint is greater than at regions of lower temperature along the length of the coil. Because evaporation of filament material such as tungsten (W) depends exponentially on the electron emission rate, the hot spot produces much tungsten evaporation, which may eventually find its way to the surface of the wafer, thereby contaminating it. In addition, the high rate of evaporation of tungsten near the hot spot reduces the operational lifetime of the coil filament.
In addition to the non-uniform evaporation of tungsten along the length of a standard pigtail filament, which can cause wafer contamination, chemical contamination of the filament itself may occur. For example, the hot tungsten filament can chemically combine with nitrogen (N), which is outgassed from a photoresist-coated wafer during implantation, to form tungsten dinitride (WN.sub.2) on its surface. Tungsten dinitride formation on the filament reduces its thermionic emissivity. To regain the desired level of emissivity, more electrical current needs to be provided to the filament, which further reduces its efficiency and lifetime.
The generally cylindrical shape of the body of a standard coil filament (i.e., circular cross section throughout the length thereof), also results in a filament which is characterized by a high thermal conductivity and heat capacity. Such a filament does not exhibit rapid thermionic emissivity changes in response to corresponding electrical current changes. Fast response time of a filament is important to be able to control the filament current during periods of outgassing.
Accordingly, it is an object of the present invention to provide a filament for a plasma shower in an ion implantation system which provides for a uniform temperature along the length thereof so as to provide a corresponding uniform thermionic electron emission characteristic, and which minimizes contamination of the filament and of the wafers being implanted by the system.
It is a further object of the present invention to provide a plasma shower filament having a low heat capacity and thermal conductivity such that its thermal emissivity is made rapidly responsive to changes in input electrical current.
It is still a further object of the present invention to provide a plasma shower filament which reduces heat conductivity from an active central portion thereof out to the filament legs, while maintaining sufficient emissive surface area of the central portion, and wherein heat generation is focused in the central portion and not conducted out through the legs.
It is still a further object of the invention to provide a filament for a plasma shower which uses less electrical power than known filaments.
It is yet a further object of the present invention to provide a filament for a plasma shower which reduces the potential for gas leaks about the interface of the filament and a plasma gas chamber in which it is mounted.